Epoxy resins represent a very broad class of organic materials that have been found useful in a variety of applications both as bulk materials in and of themselves, as components in composite materials, and as crosslinkers for various polymers. A complete discussion of this multi-billion dollar industry and the huge variety of the chemistry and products involving epoxy resins is provided in a very large amount of literature. See, for example, Flick, Ernest W. Epoxy Resins, Curing Agents, Compounds, and Modifiers. An Industrial Guide, Noyes Data Corp.: Park Ridge, N.J., 1987 and Bruins, Paul F. Epoxy Resins Technology, Interscience: New York, 1968.
Despite the large amount of available epoxy technology, the overwhelming majority of the uses of epoxy resins involve the preparation of an end product (a bulk material or a coating) that is mechanically tough, resistant to the elements, chemical resistant, and very hydrophobic. Thus, known epoxy resins have not found extensive use in hydrophilic environments in which a net hydrophilic, water-swellable, water-compatible, or readily wettable end product is required. Such uses include bio-compatible materials, aqueous separations media, coatings for lithographic printing plates, and photographic materials.
The current global trend of reducing the amounts of volatile organic compounds permitted in industrial emissions has fueled a continuing interest in the development of useful technology for aqueous-based formulations and coatings for many uses including lithography or computer-to-plate imaging members. Consequently, advances in epoxy resin chemistry have allowed for the use of epoxy resins material in aqueous formulations. The most common strategy for the use of such epoxy resins in aqueous formulations has involved the preparation of aqueous dispersions of hydrophobic, water-insoluble epoxy resins. These dispersions are typically stabilized either by a surfactant or a protective colloid such as poly(vinyl alcohol). Many of these dispersions are sold commercially and a representative example of this type of system is described in U.S. Pat. No. 5,602,193 (Stark). These epoxy resins, while dispersible in water, are still largely hydrophobic in nature and are not suitable for the applications noted above wherein the target material or coating must be highly wettable or must have a very high water uptake or water compatibility. These resins are also largely incompatible with many water-soluble polymers and multi-phase formulations may result when they are used.
There are a large number of patents describing cationic epoxy resins of the type that are commonly used in electrophoretic coating processes (also knows as electrodeposition coating). These materials, however, are fundamentally different from negatively charged epoxy materials because they have charged units that are cationic rather than anionic. The differently charged resins will thus have fundamentally different strengths and limitations in coating formulations. Specifically, both classes of resins will have different compatibilities with other formulation components.
Epoxy resins having anionic or zwitterionic moieties are broadly described in ACS Symposium Series # 114 (Epoxy Resin Chemistry), pp. 57-69, (Vol. Date 1978), Journal of Coatings Technology Vol. 54, No. 688, pp. 35-44 (1982), Journal of Coatings Technology Vol. 54, No. 686, pp. 33-41 (1982), U.S. Pat. No. 3,928,156 (Wismer et al.), and U.S. Pat. No. 4,066,592 (Wismer et al.).
These resins, however, contain carboxylate units as the anionic moieties and are not feasible for use in low pH coating formulations. In addition, it must be noted that the term “epoxy resin” is commonly used to describe materials in the art include compounds that contain epoxy moieties as well as compounds that are derived from epoxy-containing precursors that contain no epoxy moieties. The carboxylate resins described in the prior art are derived from compounds containing epoxy resins. They do not include both carboxylate and epoxy moieties in the same molecule.
Thus, there is a need for epoxy compounds that are not pH sensitive, can be readily formulated and used in hydrophilic (or aqueous environments) and do not lose their ionic charge in coated form.